The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created at the wafer surface. The need for precise wafer surfaces becomes more important as the size of the integrated circuits decreases and the number of microstructures per integrated circuit increases. Therefore, between some processing steps, it may be necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.
In an exemplary prior art polishing method, one side of the wafer is attached to a flat surface of a wafer carrier or chuck and the other side of the wafer is pressed against a flat polishing surface. In general, the polishing surface includes a polishing pad. Polishing pads can be formed of various commercially available materials such as a blown polyurethane.
During the polishing or planarization process, the workpiece (e.g., silicon wafer) is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated over the inner and outer radial surface of the polishing pad.
Additionally, in a chemical mechanical planarization ("CMP") process, a slurry, such as, for example, a water-based slurry having colloidal silica particles typically is deposited between the polishing pad and workpiece. CMP processes and apparatus are well known to those skilled in the art; such processes and apparatus will not be described in detail herein. For a more detailed description of CMP processes, see, for example, Arai, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud et al., U.S. Pat. No. 5,329,732, issued July, 1994; Karlsrud, U.S. Pat. No. 5,498,196, issued March, 1996; and Karlsrud et al., U.S. Pat. No. 5,498,199, issued March, 1996. By this reference, the entire disclosures of the foregoing patents are hereby incorporated herein.
Chemical Mechanical Planarization or Polishing (CMP) occurs when pressure is applied between the polishing pad and the workpiece being polished. The mechanical stresses and the abrasive particles within the slurry creates mechanical strain on the chemical bonds on or near the surface being polished, rendering the chemical bonds more susceptible to chemical attack or corrosion (e.g., stress corrosion). That is, after the mechanical stresses weaken the chemical bonds on the surface, the chemical agent in the slurry will attract certain atoms from the workpiece surface, thus actually removing part of the surface material. Consequently, microscopic regions are removed from the surface being polished, enhancing planarity of the polished surface.
Presently known polishing techniques are unsatisfactory in several regards. For example, during polishing, the polishing agents such as slurry and deionized water may not be deposited evenly over the entire surface of the polishing pad; such uneven polishing agent distribution can result in dry spots on the polishing pad. Consequently, the polishing effect of the pad can be non-uniform across the surface of the workpiece, resulting in a non-planar workpiece surface.
The processing of workpieces may require the deposition of multiple polishing agents from multiple sources. Each source may cause the polishing agents to distribute differently across the surface of the polishing pad. Thus, the polishing rate and uniformity of the polishing rate across the surface of a workpiece may depend on the source from which the polishing agent was dispensed.
Apparatus and methods are thus needed which will uniformly and evenly distribute polishing agents across the surface of the polishing pad and permit a higher degree of planarization and uniformity of material removed over the entire surface of the workpiece.